The present invention generally relates to airfoils, and more particularly to relatively lightweight airfoils capable of increased efficiencies when used as compressor blades of gas turbine engines.
There are ongoing efforts to increase the work per stage of compression in gas turbine engines to reduce the overall engine system cost. Such improvements can be evaluated in part by a factor known as AN2, which is the product of the area of the compressor blade inner and outer flow paths multiplied by the mechanical speed squared. Compressor blades of gas turbines are typically mechanically attached to rotor wheels/disks with a fir tree or dovetail-configured mechanical attachment, whose life is limited by the high loads that must be withstood due to the size and weight of the blades. Heavy blade airfoils require large blade attachments and create large attachment stresses, which in turn result in large disk rim loads that necessitate large disks to support those loads. Higher disk speeds necessary to increase AN2 result in still higher blade loading, requiring further increases in the size and weight of the blade attachments and disks.
In view of the above, it can be appreciated that reductions in airfoil weight would be advantageous for improving engine efficiencies and reducing costs. However, weight reductions must not be made at the expense of the structural integrity of the blade. For example, during engine operation the air flowing over compressor blades will vary in terms of speed, temperature, pressure, and density, resulting in the blades being excited in a number of different modes of vibration that induce bending and torsional twisting of their airfoils. The resulting vibration-induced stresses in the blades can cause high cycle fatigue (HCF), particularly if blades are excited at their resonant frequencies. Several technologies have been investigated to address the need for damping fan and compressor airfoils. Notable examples include visco-elastic constraint layer damping systems (VE/CLDS), air-films, internal dampers, and coatings. However, these damping technologies often encounter limitations related to structural integrity, aerodynamic efficiencies, and manufacturing difficulties.